Put out his litter box,smelly food,pieces of your clothing.
Make a chum trail with used litter or wet food in a gallon of warm water.
Keep checking garages,sheds,basements,secluded spots,under porches and vehicles. One of mine ( indoor) hid 5 days in a neighbor's basement. Had looked there several times.
Go out at night eith a flashlight and call him. Go repeatedly,especially last at night.
Alert shelters,rescue groups.groups that feed feral colonies,vets.
Check with them consistently.
Prayers he is soon home and SAFELY INSIDE.
Put up flyers everywhere asap. It will help a lot. Be sure to tell people to take a picture of the flyer so they’ll have your phone number too. Also people put their dirty laundry outside for the scent of home